routing VS nat

[James Pifer]

I'm trying to setup linux as a router instead of NAT over a VPN. Here
are the details:

I have an FC3 machine running pptpclient. 
I have pptp configured to do "LAN to LAN"
I have the FC3 machine set to act as a router:
	/proc/sys/net/ipv4/ip_forward is set to 1
After I make the connection I do iptables -F so it's wide open

When I try to use the connection the machine is still doing NAT. Is
there a way to make this box act like a router over the VPN and NOT do
NAT?

Thanks,
James